Since BC is edible, it is utilized in the food industry. BC's high dispersibility in water further provides it with a lot of industrial applications, such as to maintain viscosity of food, cosmetics or coating agents, to strengthen food materials, to maintain moisture, to improve stability of food, and to be used as low-calorie additives and as an emulsion stabilizer.
BC is characterized by a sectional width of its fibrils, which is smaller by two orders of magnitude than that of other kinds of cellulose such as those derived from wood pulp.
Owing to such structural and physical features of microfibrils, a homogenized BC has plenty of industrial applications as a strengthening agent for polymers, especially hydrophilic polymers. Products prepared by solidification of the homogenized BC in the form of a lump or paper show a high elastic modulus in tension owing to the above feature, and are therefore expected to have excellent mechanical properties for use in various kinds of industrial materials.
Methods for the production of BC are described in, for example, Japanese Patent Laid-Open Application Sho 62(1987)-265990, Japanese Patent Laid-Open Application Sho 63(1988)-202394 and Japanese Patent Publication Hei 6(1994)-43443.
As a nutrient medium suitable for the cultivation of the cellulose-producing bacteria, Schramm/Hestrin medium is known, which contains carbon source, peptone, yeast extract, sodium phosphate and citric acid (Schramm et al., J. General Biology, 11,pp.123-129, 1954). Further, it has been found that the productivity of the BC is increased by the addition of an accelerator for the cellulose production such as inositol, phytic acid and pyrroloquinoline quinone (PQQ) (Japanese Patent Publication Hei 5(1993)-1718; Mitsuo TAKAI, Japan TAPPI Journal, Vol.42, No.3, pp.237-244), carboxylic acid or their salts (Japanese Patent Laid-Open Application Hei 5(1993)-191467; Japanese Patent Laid-Open Application Hei 7(1995)-39386), invertase (Japanese Patent Application Hei 5(1993)-331491; Japanese Patent Laid-Open Application Hei 7(1995)-184677) and methionine (Japanese Patent Application Hei 5(1993)-335764; Japanese Patent Laid-Open Application Hei 7(1995)-184675) into such a nutrient medium.
Furthermore, there have been proposed a method for cultivating the cellulose-producing bacteria under a specific range of oxygen-transfer coefficient (K.sub.L a) (Japanese Patent Application Hei 7(1995)-31787), a method for cultivating the cellulose-producing bacteria while maintaining the concentration of carbon sources in a culture broth at a specific level or more (Japanese Patent Application Hei 7(1995)-267407) and a method for cultivating the cellulose-producing bacteria while maintaining the internal pressure within a fermentation tank at a specific level or more at a certain stage during the cultivation (Japanese Patent Application Hei 7(1995)-76408).
The bacteria may be generally cultured in any known culture conditions such as static culture, shaken culture, and aerated and agitated culture, and in any known culture operation methods such as batch fermentation, fed batch fermentation, repeated batch fermentation and continuous fermentation.
Means for agitation include impellers (agitating blades), air-lift fermenters, pump-driven recirculation of the fermenter broth and any combination of these means.
The impellers include gate-shape impellers, turbine impellers, double helical ribbon impellers and screw impellers.
An economical and high-yielding method for the production of bacterial cellulose is described in Japanese Patent Laid-Open Application Hei 8(1996)-33495, wherein the concentration of the bacterial cellulose in a culture medium is kept at a specific level or less or the oxygen consumption rate is kept at a specific level or more by a continuous removal of the culture medium from its culture system and a continuous supply of a fresh culture medium having almost the same volume as the removed culture broth.
The above method has overcome the disadvantages that accumulation of BC in the culture broth during the culture of the cellulose-producing bacteria will increase viscosity of the culture broth and make it difficult to supply a necessary amount of oxygen into the culture broth. As a result, a high production rate has been attained. However, since a dilution rate (i.e., a rate of supply of the medium) is relatively high, sugars will remain unused in the withdrawn culture broth, leaving a problem to be solved in the yield of BC production.
The present inventors have studied to solve the above problem so as to make the present invention.